now, here's einstein's second postulate: all the laws of physics will be seen to be the same in uniformly moving reference frames. this is moving uniformly with respect to this. in fact, einstein said, it makes no difference whether this is at rest and this is moving or this is at rest and this is moving away. motion really is relative. it's a relative motion between the planet and the ship. so, let me ask you a question. these six minute flashes that the ship really is sending-- really is sending the six, we agreed on that before-- these six minute flashes are gonna be seen how long apart by that person back here, who's now looking, who puts the telescope up and takes a look? check your neighbor. what's the answer, gang? - 12. - 12. you see that. i mean if 6 is seen as 12, then 6 will be seen as 12. so we can complete this diagram up here and say this. we have another planet here. a person watching these flashes, okay? what happens? really the flashes are like this, okay? they're all scrunched up in here and all dragged out here. and so, six minutes will be seen three for approach, but f